net,os: support converting between *os.File and net.Conn on Windows

The runtime poller and os.NewFile recently gained support for
disassociating the handle from the runtime poller IOCP (see CL 664455).
This was the main blocker for allowing the conversion between *os.File
and net.Conn.

Implementing the conversion is now trivial. The only remaining work,
implemented in this CL, is improving os.NewFile to also support
socket handles and updating some build tags so that Windows can share
almost the same net's File implementation as Unix.

There is one important limitation, though: the duplicated socket handle
returned by the various File methods in the net package is not
usable on other process. If someone needs to pass a socket handle to
another process, they should manually call the WSADuplicateSocket
Windows API passing the process ID of the target process.

Fixes #9503.
Fixes #10350.
Updates #19098.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-race,gotip-windows-amd64-longtest,gotip-windows-arm64
Change-Id: Ic43cadaac2662b925d57a9d362ddc7ae21d1b56e
Reviewed-on: https://go-review.googlesource.com/c/go/+/668195
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

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